Renewable energy options
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During the conceptual design phase of a building project, one valuable use of Building Energy Modeling (BEM) is to identify an optimal combination of energy efficiency measures and renewable energy generation that meet the project's performance goals. BEM software can estimate building energy consumption and assess the impact of design alternatives, provided that the inputs selected represent realistic operation scenarios (see Preparing Model Inputs). By combining this energy consumption information with an estimate of renewable energy generation, calculated using either BEM or a separate method, a design can be developed that meets energy performance targets.
One common question that arises during conceptual design is how large of a building-integrated photovoltaic system would be required to offset a building's expected energy use. In some cases, an early BEM analysis may reveal that the current design lacks sufficient roof area to accommodate a PV system that would provide net-zero energy performance. In this scenario, the design team could explore alternative design options that offer more space for a PV system and use BEM to evaluate energy efficiency measures that reduce energy consumption.
BEM may also be used to estimate energy cost and carbon emission impacts of building-integrated renewable energy. Many electricity tariffs include time-of-use charges, and in most regions the carbon emission factors associated with electricity generation vary by time of day and by season. Energy cost and carbon emissions can be estimated by combining hourly energy consumption results provided by a BEM model with hourly estimates of renewable energy consumption, which may be calculated by BEM software or other methods.
See also Determine Renewable Generation Objectives.
Impact of Renewable Energy Options
Renewable energy options have various types of impacts that should be considered. Below are some key impacts:
- Energy Consumption and Emissions: Net energy consumption and carbon dioxide emissions are key impacts to consider. For example, a photovoltaic (PV) system will affect the pattern of seasonal electricity usage and the shape of the daily electricity usage profile. This change will alter the time of day when peak electricity consumption occurs, impacting the building's effect on the local electricity grid. These changes in seasonal and daily load profiles will impact the carbon dioxide emissions related to the electricity consumption if time-varying carbon emission factors are considered in the calculation.
- Energy Cost: Depending on the size of the PV system relative to the building's electricity demand, the renewable energy system can have a significant impact on energy cost. The cost of energy will be affected by the change in consumption and may also be impacted by the application of a different utility tariff and interconnection fees.
- Architectural Design: The renewable energy system will typically impact architectural design. PV and thermal systems usually require roof space, but PV panels or thermal collectors can also be integrated into facades. Space may also be needed for electrical and thermal distribution systems and electrical equipment such as inverters.
- Structural Design: Renewable energy systems may affect structural design, requiring consideration for fastening and load-bearing.
- Site Design: Site design will be affected if renewable energy systems are located not on the building but instead on areas such as canopies over parking lots or ground-mounted arrays.
- Resilience: In the case of power outages, renewable energy systems combined with battery storage may affect resilience.
- Energy Code Compliance: The presence of a renewable energy system may or may not affect energy code compliance. Some energy codes allow credit for the energy produced by a renewable energy system when the building complies using a performance path. Other code compliance, such as fire codes and electrical codes, may be affected by the presence of a renewable energy system.
- Green Building Certification: Green building certification, such as LEED, may offer credit for installation of a PV system or other renewable energy systems.
- Embodied Carbon: Renewable energy systems will affect the embodied carbon represented by a building.
- Maintenance: Maintenance of a renewable energy system, such as cleaning of PV panels, may be an impact.
Renewable Energy Alternatives
Several types of renewable energy systems might be considered at the conceptual design phase, including:
- Electricity generation with a photovoltaic (PV) system (the most common)
- Thermal energy generation, typically hot water, with a solar thermal system
- Electricity generation with wind turbines
- Electricity generation with micro-hydro systems
A PV system is often the most practical and cost effective renewable energy alternative. The conceptual design phase is an excellent time to evaluate opportunities to integrate PV into the building or site design.
- Roof-mounted (most common)
- Integrated with roofing material, such as solar roof tile
- Integrated with the façade, such as spandrel in a curtain wall system.
- As part of a sun shade or awning
- Canopy over parking
- Ground-mounted
At the conceptual design phase, important design questions related to PV systems are the appropriate size and location. Many other details, such as the type of PV panel, type of inverter, and mounting system, can be developed during later design phases. The priority at the conceptual design phase is to assess impact and feasibility and to identify opportunities to integrate renewable energy into the architectural design.
For a good summary of building design strategies to maximize the area available for a PV system, along with other PV system integration guidance, see ASHRAE’s series of Achieving Zero Energy design guides[1].
Energy storage, while not a necessary part of a renewable energy system, may be appropriate to consider as an option that can affect resilience and energy cost.
Guidance on Modeling Approach
There are two relevant calculations:
- Building energy consumption estimate
- Renewable energy generation estimate
For the building energy consumption estimate, it is important to develop an energy model that reflects likely operating conditions, especially if the goal is to meet a specific EUI target or achieve net zero energy performance (see Preparing Model Inputs). This estimate can then be used for preliminary sizing of the renewable energy system.
To estimate the renewable energy generation, there are various calculation options available.
- Rules of thumb may be suitable for initial design exercises or when reliable performance data is available from similar installations nearby.
- Dedicated software for calculating renewable energy generation can be a good choice during conceptual design, especially if the goal of the analysis is to estimate a project’s net EUI. In that case, the estimated annual energy production of the renewable system can be subtracted from the estimated building energy consumption from BEM. Some dedicated software can go farther than annual energy consumption and can estimate monthly or even hourly production using typical weather data. NREL’s PVwatts[3] software is one example. That estimate of hourly electricity generation can be matched with hourly energy consumption output from BEM software to calculate net hourly energy load, using a spreadsheet or other data post-processing approach.
- BEM software with integrated renewable energy capability offers the advantage that the post-processing step can be avoided. In addition, the energy cost results can reflect the renewable energy system contribution, as long as the appropriate utility rates are used in the BEM software. However, the renewable energy calculation capabilities of BEM tools vary, so it is important to understand the method and its limitations before using this approach.
Impact of surroundings on generation potential
It is important to consider the impacts of the surrounding on solar generation potential. Trees or nearby buildings and other structures can cast shadows on the PV array which will reduce the generation potential compared to the same array with no nearby shading features.
Guidance on Presenting Results
The following presentations of results are examples of analysis and visualization that can be very useful to the design team during conceptual design.
| Climate Zone | Target EUI, kBtu/ft2-yr | PV Area as Percentage of Floor Area |
|---|---|---|
| 0A | 27.5 | 30.6% |
| 0B | 28.9 | 20.9% |
| 1A | 26 | 20.0% |
| 1B | 27.1 | 23.6% |
| 2A | 25.5 | 20.7% |
| 2B | 23.3 | 17.0% |
| 3A | 23.7 | 21.2% |
| 3B | 22.4 | 16.1% |
| 3C | 19.8 | 16.2% |
| 4A | 22.7 | 22.2% |
| 4B | 21.4 | 15.8% |
| 4C | 21.1 | 25.1% |
| 5A | 22 | 23.3% |
| 5B | 21.6 | 17.7% |
| 5C | 19.9 | 22.7% |
| 6A | 23.7 | 23.7% |
| 6B | 22.4 | 22.6% |
| 7 | 23.9 | 25.9% |
| 8 | 25.3 | 37.8% |
The data in the table above come from the Advanced Energy Design Guide for Multifamily Buildings - Achieving Zero Energy[4] and they are based on simulated energy consumption for a multi-family building in a range of climate zones.
Additional Resources
- Advanced Energy Design Guide for Multifamily Buildings - Achieving Zero Energy (2022). Guides also available for Small to Medium Office Buildings (2019) and K-12 School Buildings (2018)
- How does solar work? U.S. Department of Energy, Energy Efficiency and Renewable Energy Office.
- NREL's PVWatts® Calculator. https://pvwatts.nrel.gov/
- Building Energy Software Tools Directory. https://www.ibpsa.us/best-directory-list/
- National Aeronautics and Space Administration (NASA) Science webpage “How do Photovoltaics Work?”: https://science.nasa.gov/science-news/science-at-nasa/2002/solarcells
Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.
References
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